Viruses usually
remain infectious for several weeks at 4degC providing they are not contaminated with
bacteria or fungi. To retain stocks they are frozen at -70degC or freeze-dried. The
failure to maintain the cold chain is a common reason for vaccine breakdowns in the
tropics.

The enveloped viruses with helical nucleocapsids are the most labile, the
poxviruses and small icosahedral viruses are best able to survive in dust or fomites
(greek for inanimate wooden objects).